1. Field of the Invention
The present invention relates to compounds made by chemically bonding a host material with a guest material. These compounds are useful in an emissive layer of an organic light emitting device.
2. Description of the Related Art
Organic light emitting devices (OLEDs) typically comprise a layer of emissive material between an anode and a cathode. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons form excitons in the emissive layer to emit light.
Electrodes are chosen to facilitate charge injection. A transparent indium-tin-oxide (ITO) anode has a relatively high work function and is therefore suitable for use as a hole injection electrode, while low work function metals such as Al, Mg and Ca are suitable for injection of electrons.
To improve the power efficiency of an OLED, it is frequently desirable to enhance charge injection at the electrode interface. Hole transport layers and electron transport layers may be added adjacent the respective electrodes to facilitate charge transfer. Depending upon whether hole transport or electron transport is favored, the light emissive layer may be located closer to the anode or the cathode. In some instances, the emissive layer is located within the hole transport or electron transport layer.
Improved performance can be obtained if blocking layers are provided to block against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device. Likewise, a modifying layer may be used to improve the contact with one or both of the electrodes, or to improve the interface between two other layers.
Some of these layers can be combined. For example, a double-layered structure is fabricated from a combined hole-injecting and transporting layer together with a combined electron-transporting and light-emitting layer. Likewise, a triple-layered structure is composed of a hole-injecting and transporting layer, a light-emitting layer, and an electron-injecting and transporting layer.
Hole transport layers may include triarylamine-based materials, although many other hole transport materials are known. Likewise, an aluminum quinolinolate complex known as AlQ3 is a well known electron-transport material which has been used in OLEDs, although other electron transport materials are known.
Emissive materials having widely varied structures are known in the art and are generally selected based on color, brightness, efficiency and lifetime. These emissive materials may themselves also have electron transport or hole transport characteristics.
In addition, it is possible to form these layers from a xe2x80x9chostxe2x80x9d material doped with another material (the xe2x80x9cguestxe2x80x9d material) designed to achieve the desired effect of the layer (for example, to achieve a hole transport effect, an electron transport effect, or an emissive effect). One conventional method of dispersing the guest molecules into the host is coevaporation. In the case of an emissive guest-host system, the host must be able to transfer energy to the guest so that a maximum amount of energy contributes to emission by the guest rather than being absorbed by the host.
OLEDs have been produced that have more than one emissive layer. By using two or more emitter layers with different emission colors and adjusting the thickness of each emitter layer, the EL spectrum of the devices could be tuned to a desired color.
Occasionally, conventional methods of creating emissive layers do not yield a uniform dispersal of the guest throughout the host layer. Thus, in these emissive layers, the guest molecules may aggregate resulting in areas of the emissive layers being brighter than other areas.
Over time, as conventional emissive layers are used, they may exhibit guest molecule drifting. This also causes emissive layers to be nonuniform, resulting in uneven performance by different areas of the layer.
There continues to be a need for OLED materials exhibiting thermal stability, having bright, high purity luminescent emission, and for materials which contribute to greater luminescence per injected charge. There is particularly a need for OLED materials which solve the foregoing problems.
In an aspect of the present invention, a compound contains a host moiety chemically bonded to at least one guest moiety according to the following general molecular structure:
Hoxe2x80x94(Lxe2x80x94G)n.
Ho represents a host moiety that receives hole, electron or hole and electron recombination energy and transfers that energy to the guest moiety. G represents a guest moiety that is a photoluminescent chromophore. L represents a direct bond or a chemical moiety that links the host and guest moiety. One to four guest moieties can be bonded to the host moiety.
Suitable hosts can be, but are not limited to, substituted or unsubstituted aromatic aryl groups, heteroaryl groups, polycyclic fused groups or combinations thereof linked by single bonds, double bonds, triple bonds or heteroatoms. Preferably, the host moiety is substituted or unsubstituted biphenyl, substituted or unsubstituted binaphthalyl, substituted or unsubstituted fluorenyl, substituted or unsubstituted pyrenyl or dimers, trimers or tetramers of fluorene, binaphthalene or pyrene.
The host moiety may contain a conjugated chromophore and moieties that have hole transport, electron transport or hole and electron transport capabilities. An example of a suitable host is three pyrenyl groups each linked to a bis-spiro-indanyl group by an oxygen atom.
Preferred guest moieties are conjugated chromophores containing at least one nitrogen that have a polarized property and a dipole moment. Guest moieties may have an aromatic ring, a heteroaromatic ring, a double bond, a triple bond or a combination thereof. Examples of suitable guest moieties are stilbenyl groups, fluorenyl groups and nitrogen containing ligands chelated with transition metals, rare earth metals or lanthanide metals. These guest moieties may have substitutent groups such as arylamines or carbazolyls.
The host moieties and guest moieties can be bonded directly together or preferably have another moiety connecting them. The moiety connecting the host and guest moieties can have, but is not limited to, carbon, silicon, oxygen, sulfur, selenium, nitrogen or phosphorus or a combination thereof.
In some cases, the connecting moiety serves to interrupt conjugation between the most moiety and the guest moiety. One way to achieve this function is to have a connecting moiety that is not conjugated.
In another preferred embodiment, the connecting moiety is 5 to 100xc3x85 in length. More preferably, the connecting moiety is 10 to 50xc3x85 in length.
In another aspect of present invention, a compound is made of a host moiety chemically bonded to at least one guest moiety according to the following general molecular structure:
Hoxe2x80x94(Xxe2x80x94(R)mxe2x80x94Xxe2x80x94G)n
wherein Ho represents a host moiety that receives hole and/or electron recombination energy and transfers that energy to the guest moiety; G represents a guest moiety that is a photoluminescent chromophore; X represents O, S, CH2, SO2, or Si or a combination thereof; R represents an alkyly group, a polycyclic group, a fused polycyclic group, an aromatic group or a fused aromatic group or a combination thereof; n is a number from one to four and m is a number from 0 to 20.
Another aspect of the present invention is organic light emitting devices containing the compounds described above. In the organic light emitting device, the compounds can serve as a dopant within a host layer or can constitute an emissive layer of their own.
In a preferred embodiment of the present invention, the compound is a dopant in a host layer wherein the host layer is substantially made of a compound which has substantially the same chemical structure as the dopant""s host moiety. For example, when a compound of the formula Hoxe2x80x94(Lxe2x80x94G), is used as a dopant, the host layer is preferred to comprise a material which has substantially the same chemical structure as Ho.
The organic light emitting devices of the present invention in addition to the host-guest layer described above, may also have hole transporting layers, electron transporting layers, or both hole and electron transporting layers.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof.